More advances on sphinx and mistune upgrade

[daquintero]

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So it's mostly building and just needs some directives we don't use to be updated. I might get round to that eventually. Tests are not fully passing

However, unfortunately, because our company needs this to be deployed to PYPI for our project to build, I'm going to have to make a m2r3 in pypi I'm very sorry, but happy to take it down once all the updates I'm doing get included mostly